In a hybrid powertrain, an electric energy conversion device, for example a motor/generator, may be used to absorb and/or supply torque to improve powertrain efficiency and fuel economy performance. In one example, a hybrid powertrain may include an internal combustion engine and an electric motor connected in series to a multiple fixed-ratio transmission. In this configuration, the electric motor produces torque output, which in turn, reduces the torque load of the internal combustion engine. Moreover, by operating the internal combustion engine at a reduced torque load, fuel economy performance of the hybrid powertrain may be improved while meeting driver demand. One approach that employs this configuration is described in U.S. Pat. No. 6,835,160.
The inventors herein have recognized that the above approach may have some issues. In particular, in the above described configuration, the electric motor may be operated to improve fuel economy performance of the hybrid powertrain; however, since both the internal combustion engine and the electric motor provide torque to the input of the transmission, the transmission may transfer torque with reduced efficiency due to the amount of torque being provided to the input of the transmission. In one particular example, the internal combustion engine and the electric motor may both supply a substantial amount of torque to the input of the transmission, thus making early shifting more difficult to suitably, smoothly, and reliably transfer the torque to the final drive/wheels. As such, this may result in shifts being delayed and by shifting later (e.g. at higher engine speeds) the transmission efficiency may be lower than if the transmission is shifted earlier (e.g. at lower engine speeds).
The above issues may be addressed by, in one example, a hybrid vehicle propulsion system comprising: an internal combustion engine; a torque converter including a lockup clutch, the torque converter receiving torque from at least the internal combustion engine; a multiple fixed-ratio transmission having an input and an output, the input coupled to the torque converter; a electric energy conversion device coupled to the multiple fixed-ratio transmission output; and a control system for adjusting torque output of the hybrid propulsion system, the control system adjusting torque output of the electric energy conversion device to reduce a transmission output torque load and adjusting the multiple fixed ratio transmission to cause a transition in torque transfer responsive to adjusted torque output of the electric energy conversion device.
Thus, by adjusting the torque output of the electric energy conversion device downstream of the transmission, torque may be provided to final drive/wheels to meet driver demand while also reducing torque load at the output of the transmission. The reduced torque load at the output of the transmission may facilitate earlier shifts or transitions between the gear ratios to improve torque transfer efficiency of the transmission. In this way, efficiency and fuel economy of a hybrid powertrain may be improved while meeting driver demand.
Further, in another example, the transmission shifting and the torque converter lockup state may be adjusted based on the distribution of engine power between multiple propulsion paths in order to improve overall hybrid powertrain efficiency. In certain operating conditions, a portion of the engine power may be distributed to one propulsion path to power the electric motor (downstream of the transmission) and/or charge the battery while the remaining portion of the engine power may be transferred through the transmission in another propulsion path to provide power to the drive wheels. In these conditions, the transmission shifting and torque converter lockup scheduling may be adjusted to compensate for the reduction in net input power to the transmission. In this way, hybrid powertrain efficiency and fuel efficient may be improved while meeting the driver demand.
In still another example, a control architecture for a hybrid propulsion system may be provided that considers the tractive effort capabilities of the respective hybrid powertrain torque sources for a selected operating condition and adjusts the transmission shifting and the torque converter lockup state in order to distribute power flow accordingly. In particular, the control architecture may adjust the transmission shifting and torque converter lockup state based on the tractive effort capabilities of the electric torque sources, including the battery state of charge. In this way, hybrid powertrain efficiency and fuel efficiency may be improved while meeting driver demand.